Coolant feed for high voltage apparatus

ABSTRACT

A coolant feed for electrical apparatus having conductors at a high-voltage potential in which the coolant feed line is surrounded by a vacuum vessel in which is located a radiation shield of electrically insulating material containing a plurality of tubes arranged concentrically to the feed pipe through which a cooling medium flows, the tubes forming at least one outgoing line for the coolant in the direction of the axis of the feed pipe and at least one return line in the opposite direction thereby providing a coolant feed with low internal thermal losses and which is well suited for electrical apparatus having high conductor potentials.

United States Patent Kohler et al.

COOLANT FEED FOR HIGH VOLTAGE APPARATUS Inventors: Hubert Kohler,Eltersdorf; Fritz Schmidt, Erlangen, both of Germany SiemensAktiengesellschaft, Munich, Germany Filed: June 5, 1974 Appl. No.:476,456

Assignee:

Foreign Application Priority Data June 22, 1973 Germany 2331869 U.S. Cl.174/15 BH; 174/15 C; l74/DIG. 6 Int. Cl. H01B 7/34 Field of Search174/15 C, 15 BH, 15 R,

l74/DIG. 6; 62/338, 339, 440, 451, 443

References Cited UNITED STATES PATENTS 12/1964 Silver 174/DIG. 6

Primary Examiner-Arthur T. Grimley Attorney, Agent, or FirmKenyon &Kenyon Reilly Carr & Chapin 57 ABSTRACT A coolant feed for electricalapparatus having conductors at a high-voltage potential in which thecoolant feed line is surrounded by a vacuum vessel in which is located aradiation shield of electrically insulating material containing aplurality of tubes arranged concentrically to the feed pipe throughwhich a cooling medium flows, the tubes forming at least one outgoingline for the coolant in the direction of the axis of the feed pipe andat least one return line in the opposite direction thereby providing acoolant feed with low internal thermal losses and which is well suitedfor electrical apparatus having high conductor potentials.

10 Claims, 1 Drawing Figure COOLANT FEED FOR HIGH VOLTAGE APPARATUSBACKGROUND OF THE INVENTION This invention relates to coolant feedsforhigh voltage conductors in general, and more particularly, to animproved coolant feed with low thermal losses.

In electrical apparatus having conductors cooled to a low temperaturewhich conductors are at a high potential, a cryogenic medium must befed-in from the outside in order to cool the conductors. Arrangements ofthis nature are used, for example, with various types ofsuperconductors, such as superconducting cables, coils and machines. Insuch operation, the superconductors must always be maintained at'atemperature below their transition temperature. To do so requires acontinuous supply of cryogenic medium. It has been discovered that theonly practical cryogenic medium for cooling superconductors is helium.Thus, typically, helium is fed-in from the outside but must first bebrought to the high voltage potential of the conductor before it isallowed to 'come in direct contact therewith.

In operating a feeding-in or discharge device for-a cryogenic medium, inparticular for helium, provisions must be made to insure good thermalinsulation along with the necessary dielectric strength under alloperating conditions. That is to say, that the line leading from thecryogenic supply to the high voltage conductor to be cooled must be wellinsulated to insure that the coolant is effective upon reaching theconductor and also must be constructed with the necessary dielectricstrength because of the large potential difference between one end andthev other of the feeding arrangement. These requirements lead tocertain difficulties. Thermally well insulated lines are normallylocated in a high vacuum and are generally surrounded by metallic highlyreflecting surfaces and, in some cases, metallized plastic foils alsoreferred to as superinsulation. However, such metallic mirror-coatingand metallized plastic foils-are not usable in a coolant feed apparatusat high voltage potential since they couldlead to short circuits of thehigh voltage. Mirror-coatings using semiconductor materials generallyare also not sufficient since the highest reflectivity which can beobtained from such materials is only 40 to 45 percent for example, inthe case of germanium or silicon One proposed solution to the problem isdisclosed in U.S. Pat. No. 3,522,361 in which arrangement several normalconductors are led throughseveral cooling chambers each representing acooling stage between room temperature and the superconductortemperature. The cooling stage with the lowest temperature level iscooled by liquid helium. Therein, the normal conductors are joined tosuperconductors of a cable. The conductors which are designed for a highvoltage potential are disposed between the individual cooling stages ininsulator bodies which are required to prevent flash-over between theouter parts of the feed arrangement at ground potential and theconductors. The helium bath of the last cooling stage which also is usedfor cooling the superconductors in the connected cable is replenishedthrough a pipe. This pipe is concentrically enclosed by a larger pipethrough which evaporating helium from the bath or the cable canescape-The pipes are made of insulating material. Because of therelatively short length'of the feed line for the coolants, particularlythe one for the liquid helium, the current and coolant arrangement has adielectric strength which is quite low, i.e., that of helium. As aresult, the operating voltage of the connected cable must then be chosenat a relatively low value. Furthermore, this arrangement has rather highcoolant losses.

Thus, it can be seen that there is a need for an improved coolant feedarrangement which is suitable for electrical apparatus having highconductor potentials with the conductors maintained at cryogenictemperatures and which feed arrangement has low internal thermal lossesand meets the other requirements noted above.

SUMMARY OF THE INVENTION The present invention solves this problem byplacing in a vacuum vessel which has a cylinder length in the axialdirection of the feed pipe determined by the dielectric strength ofitsouter cylinder surface a radiation shield of electrically insulatingmaterial through which a cooling medium flows in several tubes arrangedconcentrically to the feed pipe and to the cylinder surface of thevacuum vessel. These tubes form at least one outgoing line for thecooling medium in the direction of the axis of the feed pipe and atleast one return line in the opposite direction.

This arrangement achieves particular advantages. Complicatedarrangements for generating a breakdown-proof potential gradient in theelectrical apparatus itself are not necessary since the coolant is fedto the electrical apparatus at the high voltage potential of theconductors. The dielectric high voltage strength of the feedingarrangement is insured by a sufficient length of the feed pipe. Thus, inaccordance with the preferred embodiment of the invention, the coolantfeedpipe has a length corresponding at least to the length of the vacuumvessel which is designed in order to obtain the required breakdownresistance at its outer cylinder surface. Since this breakdown strengthis always lower than that of the coolants in the feed pipe, adequatedielectric strength of the cooling carrying section is then alwaysassured.

BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE is a longitudinalcross section in schematic form illustrating a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The FIGURE illustrates anarrangement for feeding a coolant such as helium into an electricalapparatus such as superconducting cables, coils or machines in which theconductors are at high potential. This coolant designated A and which ispreferably helium is contained in the liquid state in a cooling tank 2.Connected to the cooling tank through a pressure reducing valve 4 is apressure bottle 3 containing gaseous helium. The pressure of the gaseoushelium on the surface of the liquid helium in the tank 2 causes theliquid helium A to flow from the coolant tank through a connecting pipe5, a corrogated pipe section 7 and then through a feed pipe 8 with anend piece 9 to an electrical apparatus not shown on the FIGURE.

The feed pipe 8 is made of an insulating material. Within it the heliumA traverses a potential gradient starting at ground, i.e., the tank 2will be at ground potential, until it reaches a high voltage potentialat the end piece 9. As a result, the conveyed helium A can then be fedthrough this end piece directly to the electrical apparatus withoutfurther need for devices to establish a breakdown proof potentialgradient. The feed pipe 8 along with its end piece 9 and corrugated pipesection 7 are enclosed within an outer pipe 10. This outer-pipe 10 isthe vacuum vessel. Pipe or vacuum vessel 10 is evacuated in conventionalfashion through a A 1 l8 and 1 l9. vessel 10 is a radiation shield 12concentric therewith.

The radiation shield 12 encloses the feed pipe over its entire length.In the illustrated embodiment, the radiation shield comprises a tripletube with two annular cross sectional areas surrounding each otherconcentrically. It is made, of an electrically insulating material andcontains an inner zone 13 bounded by an inner tube 14 and a middle tube15 representing one flow space for a cooling medium B. Another flowspace 17 ,is situated between vthe middle tube 15 and the outer tube 18.As illustrated, a second cryogenic medium B is fed into the inner zoneor flow space 13 through a tube connection 19 at the lower end which isat ground potential. The medium E rises in the flow space 13 and isconducted at the upper high voltage end 16 into the outer flow space 17returning in the latter to the bottom. At the bottom, it leaves througha connection 20 at the same height as connection 19. The cryogenicmedium E which flows through the radiation shield 12 may be liquidnitrogen at about 77K.

By causing this flow therethrough, the shield prevents heat conductionor radiation from the outside to the heliurncarrying feed pipe 8. Themedium B fed into the radiation shield 12 at connection 19 which is atground potential, during its rise, traverses a potential gradient withinthe flow space 13 up to the high voltage potential at the end 16 andagain traverses this gradient downward until it is again at groundpotential when it flows back out through the outlet connection 20.

The feed pipe 8, the radiation sield 12 in the form of a triple tube andthe vacuum vessel 10 enclosing these, can be made of glass, quartz oralso ceramics. A particularly suitable material is molybdenum leadglass. Also suitable are plastic materials which give off little gas ina high vacuum. In order to securely position the feed pipe 8, theradiation shield 12 and the outertube 10 with respect to each other,support members 21 are provided. These may be in the form of projectionsand may be made of one of the above-mentioned insulating materials.Through this arrangement, the vacuum vessel 10 which is at normaltemperature is thermally insulated by the support members 21 from theradiation shield 12 which is at the temperature of the cryogenic mediumB and from the feed pipe 8 carrying the coolant A. As a result, frostingof the vacuum vessel 10 and an attendent reduction of its dielectricstrength is prevented.

In the feed arrangement of the present invention, provision is-made thatthe feed pipe 8 is sufficiently long to insure an adequate dielectricstrength of the coolant section. Such will always be the case if the 5length of the feed pipe 8 approximately corresponds to the length of thevacuum vessel 10. As noted above, the length of the vacuum vessel 10 isdetermined by the dielectric strength of its outer cylinder surface inair. Since the dielectric strength of the feed pipe 8 carrying 10 thecoolant A and of the radiation shield 12 carrying the coolant B isalways better in a vacuum than in air [the dielectric strength of liquidnitrogen, for example being about 30 percent better than that of liquidhelium and considerably better than that of gaseous helium] 5 thisrelationship between the length of the feed pipe and the length of thevacuum vessel will always insure adequate dielectric strength. Thus, thelength of the vacuum vessel 10 and thus that of the radiation shield 12and feed pipe 8 is determined by the dielectric 20 strength of the outercylinder surface of the vacuum vessel 10 in air. It will be recognizedthat the different shrinkage of the material of the tubes, particularlythat of the radiation shield 12 with respect to the feed pipe 8 and thevacuum vessel 10 must be taken into consid- 25 eration. In order to makeprovisions for such effects,

30 It is advantageous if corona shields are provided at both ends of thearrangement. On the FIGURE, a single high voltage potential coronashield 23 is shown at the top of the arrangement as an example.

It is also advantageous to provide a separate helium pump for pumpingthe liquid helium A from the coolant tank to the feed pipe 8 through theconnecting pipe 5 and corrogated pipe 7. Such a pump 24 is shown on theFIGURE and as illustrated, can be placed in the coolant tank 2. In somecases, a pump 24 will be capable of generating sufficient pressure totransport the helium A at the required flow rate in which case thepressure bottle 3 and pressure reducer 4 can be eliminated.

With the illustrated arrangement, not only helium which is a preferredcryogenic medium, but any cyrogenic medium can be supplied to electricalapparatus at high voltage potential. Not only is the medium conducted tothe electrical apparatus in a thermally and electrically insulatedmanner but the apparatus at the same time bring the medium from groundpotential to the high voltage potential of the electric apparatus. Whenfeeding and discharging helium, a number of operating states arepossible as follows: Boiling helium, liquid undercooled helium at apressure higher than 1 atm, supercritical helium at a pressure higherthan 2.3 atm, and also gaseous helium in the temperature range of up toabout 10 K. Of these operating states, gaseous helium has the lowestdielectric strength as is well known in the art. The danger of areduction of the dielectric strength becomes particularly great if thetransfer of the coolant is carried out in the illustrated feedarrangement in an intermittent manner rather than continuously. Such mayoccur, for example during the replenishment of ing operations so that,upon resumption of the pumping of the boiling helium, a gas pulse isfirst pushed through the feed arrangement, the gas pulse being made ofrelatively warm helium gas. As is well known, the dielectric strength ofgaseous helium is reduced. considerably with increasing temperature.This possibility leads to the requirement of good thermal insulation aswell as the necessity for good dielectric strength under all operatingconditions during feeding.

As long as the helium A flows continuously through the feed arrangementin any of the above mentioned states, no difficulties can occur sincethe heat radiated into the helium carrying feed pipe from the radiationshield 12 filled with liquid nitrogen is very small. As a result, thetemperature of the liquid or gaseous helium is increased only to theslighest possible extent because of the support elements 21 which havelow heat conductivity and are disposed at a large distance from eachother. However, even for the above mentioned case where a gas pulse ispushed through the feed with the pulse made up of relatively warm heliumgas, a helium carrying pipe which has a length equal to that of theouter air distance of the vacuum vessel providesa sufficient margin ofsafety. During intermittent operation the normally helium carrying feedpipe can at most warm up to the temperature of the nitrogen because ofthe surrounding triple tube of the radiation shield 12 which is nitrogencooled and is always kept filled. At this temperature, the dielectricstrength of the helium is still about 2.5 times that of helium at roomtemperature. To further take into account problems of this nature, it isalso advantageous to increase the dielectric strength by dividing thefeed pipe 8 into individual capillaries connected in parallel for theflow of helium gas or liquid.

It should be noted that in principle, it is also possible to use liquidhydrogen rather than liquid nitrogen for thermal shielding. Sinceapproximately the same breakdown values exist for hydrogen as fornitrogen, this substitution results in no limitations with regard to theinwardly directed radiated heat and heat conduction.

Thus, an improved coolant feed for high voltage conductors in anelectrical apparatus has been shown. Although a specific embodiment hasbeen illustrated and described, it will be obvious to those skilled inthe art that various modifications may be made without departing fromthe spirit of the invention which is intended to be limited solely bythe appended claims.

What is claimed is:

l. A coolant feed arrangement for electrical apparatus having conductorswhich are to be cooled to a low temperature and which are at a highpotential comprismg:

a. a feed pipe made of an electrically insulating material forconducting a first coolant from a coolant source to the electricalapparatus;

b. a vacuum vessel made of insulating material, having a cylinder lengthin the direction of the axis of the feed pipe determined by thedielectric strength of its outer cylinder surface, surrounding said feedpipe concentrically; and

c. a radiation shield made of electrically insulating material andincluding a plurality of concentric tubes through which a second coolantcan flow, said radiation shield being arranged concentric to said feedpipe and said vacuum vesseL-and having at least one inlet line and onereturn line for coupling said second coolant into and out of saiddirection.

2. A coolant feed according to claim 1 and further including a firstcoolant and means to supply said first coolant and wherein said firstcoolant supplied is helium.

3. A coolant feed according to claim 1 and further including a secondcoolant and means supplying said second coolant and wherein said secondcoolant for said radiation shield is a cryogenic medium having atemperature higher than the temperature of said first coolant.

4. A coolant feed according to claim 1 wherein said feed pipe, radiationshield and vacuum vessel are made of one of the group consisting ofglass, quartz and ceramics.

5. A coolant feed according to claim 1 wherein the material of whichsaid feed pipe, radiation shield and vacuum vessel are made, is aplastic material which gives off little gas in a high vacuum.

6. A coolant feed according to claim 1 wherein said radiation shield hasa high voltage side and a side at ground potential and wherein saidradiation shield comprises a. an inner tube;

b. a middle tube; and

c. an outer tube to thereby form inner and outer flow passages betweensaid tubes with the inner flow passage connected to said inlet line, theouter flow passage connected to said outlet line and said inner andouter passage connected with each other at the high voltage side of saidradiation shield.

7. A coolant feed according to claim 6 wherein helium is the coolant.

8. A coolant feed according to claim 7 wherein the second cooling mediumfor said radiation shield is a cryogenic medium having a temperaturehigher than the temperature of said first coolant.

9. A coolant feed according to claim 8 wherein said feed pipe, radiationshield and vacuum vessel are made of one of the group consisting ofglass, quartz and ceramics.

10. A coolant feed according to claim 8 wherein said feed pipe,radiation shield and vacuum vessel are made of molybdenum lead glass.

1. A coolant feed arrangement for electrical apparatus having conductorswhich are to be cooled to a low temperature and which are at a highpotential comprising: a. a feed pipe made of an electrically insulatingmaterial for conducting a first coolant from a coolant source to theelectrical apparatus; b. a vacuum vessel made of insulating material,having a cylinder length in the direction of the axis of the feed pipedetermined by the dielectric strength of its outer cylinder surface,surrounding said feed pipe concentrically; and c. a radiation shieldmade of electrically insulating material and including a plurality ofconcentric tubes through which a second coolant can flow, said radiationshield being arranged concentric to said feed pipe and said vacuumvessel, and having at least one inlet line and one return line forcoupling said second coolant into and out of said direction.
 2. Acoolant feed according to claim 1 and further including a first coolantand means to supply said first coolant and wherein said first coolantsupplied is helium.
 3. A coolant feed according to claim 1 and furtherincluding a second coolant and means supplying said second coolant andwherein said second coolant for said radiation shield is a cryogenicmedium having a temperature higher than the temperature of said firstcoolant.
 4. A coolant feed according to claim 1 wherein said feed pipe,radiation shield and vacuum vessel are made of one of the groupconsisting of glass, quartz and ceramics.
 5. A coolant feed according toclaim 1 wherein the material of which said feed pipe, radiation shieldand vacuum vessel are made, is a plastic material which gives off littlegas in a high vacuum.
 6. A coolant feed according to claim 1 whereinsaid radiation shield has a high voltage side and a side at groundpotential and wherein said radiation shield comprises a. an inner tube;b. a middle tube; and c. an outer tube to thereby form inner and outerflow passages between said tubes with the inner flow passage connectedto said inlet line, the outer flow passage connected to said outlet lineand said inner and outer passage connected with each other at the highvoltage side of said radiation shield.
 7. A coolant feed according toclaim 6 wherein helium is the coolant.
 8. A coolant feed according toclaim 7 wherein the second cooling medium for said radiation shieLd is acryogenic medium having a temperature higher than the temperature ofsaid first coolant.
 9. A coolant feed according to claim 8 wherein saidfeed pipe, radiation shield and vacuum vessel are made of one of thegroup consisting of glass, quartz and ceramics.
 10. A coolant feedaccording to claim 8 wherein said feed pipe, radiation shield and vacuumvessel are made of molybdenum lead glass.